This invention is generally in the field of optical measurement techniques, and relates to a method and system for film characteristics measurements in patterned structures. The invention is particularly useful in the manufacture of semiconductor devices.
A semiconductor wafer presents a patterned article, the pattern being in the form of multiple-layer stacks. Various manufacturing steps in the manufacture of semiconductor devices require measurements of thickness or other characteristics of an uppermost or other layer of a semiconductor wafer.
Usually, the thickness measurements are carried out on predetermined sites on the wafer, each containing a known layer stack, after performing wafer alignment utilizing a so called xe2x80x9calignment featurexe2x80x9d. In other words, the knowledge of the layers"" materials and thicknesses in the stack undergoing measurements, and the location of the alignment feature, are two inherent conditions, constituting the so-called xe2x80x9crecipe designxe2x80x9d, for performing the thickness measurements. The alignment feature is a unique feature (pattern element) which is present in each of the wafer dies. To locate the alignment feature, a suitable pattern recognition technique is used, which location includes coordinates of the alignment feature and its relative location to any selected site (i.e., the die""s dimensions and the space between two locally adjacent dies). This enables location of each site to be measured relative to the alignment feature.
Unfortunately, in cases when such a recipe design cannot be automatically prepared on-line, i.e., concurrently with the measurement procedure, the wafer is removed from the production line, and the entire process is stopped waiting for the off-line preparation of the recipe design.
Attempts have been made for eliminating the need for recipe design automatic preparation and pattern recognition required therefor. U.S. Pat. No. 5,900,633 discloses a measurement technique, which utilizes a xe2x80x9clarged spot (measurement area) illumination (1-10 mm). This technique enables to obtain an average thickness value and does not provide for local measurements of each feature within the spot.
There is accordingly a need in the art to facilitate measurements in patterned articles by providing a novel method of optical measurements and a system for carrying out the same.
The term xe2x80x9cpatterned articlexe2x80x9d signifies an article formed with regions having different optical properties with respect to incident radiation. The regions of the wafer having non-patterned structure could be considered as a particular case of patterned structures characterized by an infinitive cycle as compared to the measurement spot diameter. It is a major feature of the present invention to provide such a method that enables the automatic (on-line) preparation of a recipe design associated with a specific article undergoing measurements. The term xe2x80x9crecipe designxe2x80x9d used herein signifies a computer file containing the full information required to characterize a specific site of the article. The site includes a stack of different layers, which may and may not include different locally adjacent sub-stacks (features of the pattern). The information contained in the recipe design thus includes data indicative of the layers"" thickness, materials and geometric details (e.g., wafer""s features dimensions), optical model(s) to be used for measurements in this site and interpretation data (algorithms), die size, alignment feature location, etc. This recipe design, once prepared, can be used for measuring one or more sites in the article to be measured.
It is a further feature of the present invention to provide such a method that enables mapping of tile layer thickness (or other characteristic) or mapping of the layer non-uniformity over the entire article or within a specific region (e.g., a region of a die) with desired, substantially high resolution (e.g., several micrometers). Such a mapping procedure does not necessarily require the recipe design to be previously prepared. On the contrary, the recipe design is prepared concurrently with the mapping procedure.
The main idea of the present invention is based on the use of a library (reference data), containing data corresponding to various sites within an article of a specific kind and/or various kinds, to optimize an optical model for performing the thickness measurements in a desired, arbitrarily chosen site. The reference data includes predefined solutions, including materials and/or layer stacks and/or geometric details for a certain process level covering as much as possible the expected variants of measurement sites that can appear at this process step. Generally speaking, the data corresponding to a specific site presents a set of the site""s parameters including the following: the number of layers; optical characteristics (defined by the layer materials and thickness values), geometric characteristics (feature""s dimensions), etc. The optical model includes the site""s parameters as the optical model variables. By taking at least one measurement in a specific site, and analyzing the reflection spectrum obtained using at least one of the library solutions (e.g., sequentially varying the optical model and optimizing the desired parameter to be measured until a predetermined criteria is met for each of the library constituents), the measurements in this site and/or its recipe design preparation can be automatically performed. Additional adjusting of other optical model parameters, such as optical coefficients of a selected layer material, could be carried out for further optimizing the optical model.
The present invention takes advantage of the technique of optical thickness measurements in a patterned structure that has been developed and disclosed in the U.S. Pat. No. 6,100,985 assigned to the assignee of the present application. This U.S. Patent is therefore incorporated herein by reference. This technique enables a suitable optical model for thickness measurements to be applied in a corresponding article""s patterned site.
Initially, the reference data including data records corresponding to each stack (site) of the article is created. The data record includes sets of the various stacks"" parameters, on which the optical model is based and interpretation algorithms each for a corresponding one of the parameters"" sets. The reference data is prepared by measuring a xe2x80x9cgoldenxe2x80x9d article, or other article, provided it contains all or most of the representative stack structures (or similar design rules or geometry) of the article to be measured.
It should be noted that, owing to the fact that such a multiple-layer, multiple-stack patterned article is typically manufactured layer by layer, thickness measurements are performed separately after each manufacturing step. Consequently, the reference data is designed in accordance with the manufacturing process. Usually, the data indicative of each manufacturing step (layer) is organized in sub-folders or sub-directories. Thus, the reference data (libraries) includes information on each potential measurement site on the article.
The following information about each stack (site) is included in the reference data:
algorithms suitable for performing a fitting procedure enabling calculation of the thickness or optical characteristic of uppermost or other layer(s) of the article;
the thickness values range to be used in the fitting procedure;
optical model geometric related parameters (characteristics), e.g., A, B, C, D and E;
the feature width and the space between two or more adjacent features (i.e., lines and spaces);
in the case that the site-stack is composed of locally adjacent sub-stacks, the information regarding these sub-stacks, namely materials (n and k, i.e., the index of refraction and the extinction coefficient, respectively), the nominal thickness value and its allowable tolerances towards maximal and minimal values for each layer of the sub-stack.
Thus, according to one aspect of the present invention, there is provided a method for measuring at least one desired characteristic of a patterned article containing a plurality of different pattern elements located at different sites, each including a stack of layers and presenting at least one feature of the pattern, the method comprising the steps of:
providing an optical model based on a set of parameters corresponding to predetermined characteristics of tie article, wherein said set includes inter alia parameters corresponding to geometrical characteristics of tie pattern elements, said optical model being capable of generating theoretical data indicative of spectral response of the article;
preparing reference data containing a plurality of sets of parameters for at least some of said different pattern elements;
taking a spectral measurement of light response at a selected site of said patterned article and generating measured data;
optimizing the optical model by varying said parameters"" sets; and
analyzing the theoretical spectral responses obtained through the optimization and the measured data, in order to determine therefrom said at least one desired characteristic.
The term xe2x80x9cfeaturexe2x80x9d used herein signifies at least one of the following: the dimensions of the pattern element, the space between two adjacent pattern elements and optical properties of the pattern element (defined by the layers materials and thickness).
According to another aspect of the present invention, there is provided a system for optical measuring at least one desired characteristic of a patterned article containing a plurality of pattern elements located at different sites, each including a stack of layers and presenting at least one feature of the pattern, the system comprising:
a measuring unit including a spectrophotometer that determines spectral response of a measurement site on the article and generates spectral measured data;
a memory for storing an optical model based on a set of parameters corresponding to predetermined characteristics of the article, wherein said set includes inter alia parameters corresponding to geometrical characteristics of the pattern elements and said optical model is capable of generating theoretical data indicative of spectral response of the article, said memory storing a reference data containing a plurality of sets of parameters for at least some of said different pattern element;
a processing unit coupled to the measuring unit for receiving the spectral measured data, optimizing the optical model by various parameters"" sets contained in the reference data, analyzing the theoretical spectral responses obtained through the optimization and the measured data, in order to determine therefrom said at least one desired characteristic.
According to still another aspect of the present invention, there is provided an optical profilometer for determining a surface profile of a patterned article containing a plurality of pattern elements located at different sites, each including a stack of layers and presenting at least one feature of the pattern, the system comprising:
a measuring unit including a spectrophotometer that determines spectral response of a measurement site on the article and generates spectral measured data;
a memory for storing an optical model based on a set of parameters corresponding to predetermined characteristics of the article, wherein said set includes inter alia parameters corresponding to geometrical characteristics of the pattern elements, and said optical model is capable of generating theoretical data indicative of spectral response of the article, said memory storing a reference data containing a plurality of sets of parameters for at least some of said different pattern element; and
a processing unit coupled to the measuring unit for receiving the spectral measured data, optimizing the optical model by various parameters"" sets, analyzing the theoretical spectral responses obtained through the optimization and the measured data, in order to determine therefrom the thicknesses at selected sites, and calculating the surface profile from said thicknesses.